Recently fluorescence endoscopies such as light-induced fluorescence endoscopy and photodiagnosis have been developed in order to detect lesions occult to routine white light endoscopy. These fluorescence endoscopies are performed with the combination of fiberscope and high-sensitive fluorescence camera, and are thus clumsy to operate and their price is very expensive. Moreover, their fluorescent images are quite dim.
Fluorescein sodium is a fluorescent substance daily used in ophthalmology to reveal fine vascular structures of the retina. In the method of this fluorescein fundus photography, the exciter filter is used to select excitation light from light source and the barrier filter cuts all excitation light reflecting from the fundus but passes through all fluorescent light emitted. In order to a get sharp image, a multiple-layered interference exciter filter which strictly selects only excitation blue light with the most potency to excite fluorescein sodium is recommended.
During a short period of 1975-1976, using a fiberscope, Katsu et al succeeded in observing and recording fluorescence of fluorescein sodium on the stomach surface and noted that this fluorescence endoscopy could determine the extent of gastric cancer in the mucosa. However, this fluorescence fiber endoscopy had never been performed since then. According to Katsu, the reasons were as follows.
Firstly, fluorescein fiber-endoscopy had some problems in recording the findings. The images were so dim that it was necessary to force-develop (4 to 8 times) high sensitive films (ASA 160) used. The exciter filter used by Katsu et al. was fluorescein isothiocyanate (FITC) interference filter which selectively passes only the excitation blue light with a peak at 495 nm.
Secondly, that electronic endoscopy, which allows a physician to confirm findings at the time of examination, became popular was another reason that fluorescein fiber-endoscopy disappeared. This is because the FITC interference filter, which was recommended as exciter filter, did not function with an electronic endoscope. It gave even dimmer images being used with an electronic endoscope than used with a fiberscope. The way to solve this problem was thought to be using a more powerful light source but this has never succeeded. An FITC interference filter cannot stand the heat of the electronic endoscopic light source.
Thirdly, using a fiberscope, one can easily insert the barrier filter in front of the camera and take it away outside the body. But, using an electronic endoscope, the barrier filter has to be placed in front of the CCD located in the tip of the scope and it is very difficult to insert and remove the barrier filter while the electronic endoscope is inside the body. In order to do both routine and fluorescence electronic endoscopies, the scopes with and without the barrier filter have to be inserted into the body two times. This means two times of pain for the patient.
In 1993, Tada et al. analysed the change of electronic colonoscopic images after administration of fluorescein sodium by computer. In this study, electronic endoscopic images were sequentially recorded as digital data and processed by computer to accentuate the change with fluorescence. In other words, they did not succeed in observing the real time fluorescent images by electronic endoscopy.